Recently, a single crystal nitride semiconductor has been considered to be one of the most important materials in the optics-related industry field due to many attempts to develop photoelectronic devices such as light emitting diodes (LED) spanning from blue/green to (near) ultraviolet rays, laser diodes, and photosensors, and due to rapid technical progress of the photoelectronic devices. First of all, practical photoelectronic devices including a nitride semiconductor are mainly grown on a dielectric sapphire substrate and conductive silicon carbide (SiC) at a high temperature of 1200 degrees Celsius or above under a hydrogen ambient using ammonia (NH3) and hydrogen (H2) as a carrier gas. However, dielectric sapphire and conductive silicon carbide substrates are much more expensive than silicon (Si) materials, and therefore their effectiveness is lowered in terms of economics. In addition, the nitride photoelectronic devices manufactured on the dielectric sapphire substrate should smoothly discharge heat since much heat is generated during their operation, but sapphire has a crucial drawback in that it has a significantly poor thermal conductivity.
In addition to the dielectric sapphire and silicon carbide substrates, transparent conductive zinc oxide (ZnO) has been in the spotlight as a substrate for next-generation nitride light-emitting devices due to its lattice constant with a nitride semiconductor, good electrical and thermal conductivity, excellent light transmittance, and low cost. However, these transparent conductive zinc oxides (ZnO-based oxides) are easily decomposed since a substrate surface made therefrom is decisively unstable at a high temperature of 600 degrees Celsius or above under a high vacuum of 10−3 Torr or more, and it is also nearly impossible to allow a single crystal nitride semiconductor to grow at a high temperature of 800 degrees Celsius or above under a reducing ambient such as hydrogen (H2) or ammonia (NH3) since the conductive zinc oxides are more actively reduced under the reducing ambient.
As other conductive substrates, silicon (Si), silicon germanium (SiGe), and gallium arsenide (GaAs) have been in the spotlight. The conductive substrates are deformed/decomposed at a high temperature of 500 degrees Celsius or above due to motion of a dislocation slip system present in their substrate, and it is also difficult to allow a superior nitride thin film to grow due to a large lattice constant with a nitride semiconductor and a large thermal expansion coefficient.